Mac Pro Tower - Sapphire Radeon Rx580 is really, really slow
Unfortunately Apple has deprecated OpenCL and OpenGL in favor of their own Metal API. In fact, Blender has recently dropped OpenCL support on macOS completely, due to too many bugs in the macOS OpenCL compiler.

Long story short, due to apple's rejection of widely used open APIs, it's unlikely there will be any support for OpenCL rendering on macOS in the immediate foreseeable  future. Perhaps someday it will be supported via vulkan, but in the meantime it looks like you're out of luck/stuck on old versions of blender.

If you don't want to shell out the $$$ to dual-boot Windows, you might try running Linux. Historically, Linux has outperformed Windows when it comes to CPU rendering (it usually matters less for GPU rendering, but it's often a little faster there too). That said, I was unable to find any up-to-date benchmarks comparing the two, so take this with a grain of salt.